Sorry, folks, but those of you who advise writing/contacting a premises AFTER the event are missing the point. It has to be done at the time, on the spot, otherwise the chances of getting any form of satisfaction is virtually nil - and, after all, that's what any reasonable person would expect, isn't it?
You must do it at the time for it to have the maximum impact, i.e. that you are genuinely dissatisfied, the fact that you are indeed serious about complaining, and, last but not least, the chances that there are other people present, for if so, the reputation of the business is put in even more jeopardy in front of other witnesses.
Do not delay is my advice, or else you may as well not bother, particularly where restaurants/cafes etc are concerned.